Witches Broom - problems

[vincentnm]

Hi All,

Captured on 30th June and 1st July. I put in a lot of effort for this. Two nights of capture and over 2 weeks of intermittent processing. At the end of it all, I am disappointed with the image. Here are my list of problems:

1) There was significant vignetting on 600s subs. I thought that the hyperstar was supposed to deliver a flat field. But not in my case. Anyone got a clue why? Tried taking flats for the first time and that was a effort in itself. Took over 100 flats in 3 separate sessions, both T-Shirt flats and Twilight flats. Finally ended up ended up choosing 20 from the T-Shirt session. Colour flats did not work well. So desaturated the colour converted ones (Talk about going full circle)!

2) The second problem was horrendous skyglow. I dont have a light pollution filter yet. Many of the excellent Veil pictures here are with narrowband filters. So I doubt whether this object will stand up to singleshot colour without a LPS/CLS filter.

3) I did manual collimation. It is not good enough. I guess software assisted collimation is a must if I am imaging at f1.8. Downloaded CCD Inspector as suggested by Steve. Yet to see a cloudless/rainless night to try it out.

All in all - so much effort, so little image quality, but quite a lot of learning. Comments and suggestions welcome please.

Imaging Scope: Celestron8 with Hyperstar3

Imaging Camera: Starlight Express M8C

Guidescope: Skywatcher ST80

Guidecamera: Imaging Source DFK

Mount: EQ6

Exposure: 19 X 10 minute subs

Processing: Flats/Bias calibrated and stacked in DSS, Histogram stretched and Unsharpmask in PS, Various Noels Actions, Gradient Exterminator

Thanks,

Vincent.

[SteveL]

I thought that the hyperstar was supposed to deliver a flat field

It does give a very flat field, but what you are talking about is field illumination issues aka vignetting. These are two seperate issues.

[Starstrike]

Hi Vincent,

I recently had problems collimating my C8, so I used the live view of my canon 1000d (via a computer) on vega and went to zoom, a few tweeks on the adjustment screws and success. I see you have a canon 350d.

Good luck, Paul

[Aza]

If you dont have a LP filter, 10 minutes is a looong time!

Even with the filter I would think you dont need anywhere near 10 mins with a hyper star set up.

[minimalist]

I'd also agree with Arran, imaging at that fast a ratio will hit skyglow very quickly. What's even harder is controlling the brightness of the starfield and bringing out the comparably very faint nebula... this is like M31, a big dynamic range probllem. The result looks pretty good to me though.

Daniel

[vincentnm]

It does give a very flat field, but what you are talking about is field illumination issues aka vignetting. These are two seperate issues.

Thanks for clarifying that Steve. I read up on Field Curvature and Vignetting. Field curvature if caused by the curved nature of a focal plane. Vignetting has many causes, most convincing was the one about the angle of incidence of light rays on a CCD sensor. Slanting angles (away from the centre) register less photon hits.

A field flattener like that of a hyperstar lens can correct the curved focal plane, but it cant cure the slanting angle of incident light rays on a flat sensor.

Anyone making a curved sensor, engineered for a particular scope? Could solve quite a number of optical problems.

Thanks for your comemnts on skyglow guys. I am convinced that skyglow was the dominant problem with this image. I am on the lookout for a 2" CLS or UHC filter.

I dont have the heart to throw out all this data. I will try dynamic background extraction in PixInsight and re-process. Expect a new image soon. Any other ideas to remove bad skyglow?

Thanks,

Vincent.